1. Field of the Invention
The present invention relates to a source follower circuit, a laser driving apparatus, semiconductor laser apparatus, a current-voltage conversion circuit, and a light receiving circuit.
2. Related Background Art
A conventional source follower circuit comprises a first field effect transistor and a second field effect transistor. Each of the first and second field effect transistors has its source, drain and gate. In the first field effect transistor, the gate is connected to the input of the circuit, the drain is connected to a grounded wiring, and the source is connected to the output of the circuit. In the second field effect transistor, the source is connected to a negative power line for the circuit, and the drain is connected to the output. A predetermined bias voltage is supplied from a biasing stage to the gate of the second field effect transistor.
Recently, small transistors are used in semiconductor integrated circuits, such as a source follower circuit. The inventor found the following problem: the smaller the transistors in the source follower are, the worse the constant current characteristics of a current source circuit becomes. The inventor conducted investigation to improve the characteristics of the source follower circuit.
In this investigation, the inventor noted the following. In order to increase the operating speed of a semiconductor integrated circuit containing III-V compound semiconductor transistors, the channel length of the compound semiconductor transistors is shortened. However, the shortening of channel length degrades saturation in the drain current characteristics of compound semiconductor transistors. If the saturation characteristics of the drain current are utilized for obtaining a constant current source, the shortening of channel length decreases the gain of the source follower circuit.
The inventor then conducted further investigation. In order to maintain the good constant current property of the current source containing the compound semiconductor transistor, the channel length of compound semiconductor transistors can be lengthened in the current source. However, if compound semiconductor transistors have respectively different channel lengths, one compound semiconductor transistor has a threshold voltage different from others in the semiconductor integrated circuit. This variation in the threshold voltages is not preferable in terms of designing of the source follower circuit. Namely, what is desired is a circuit design approach to improving the current source characteristics of the source follower circuit.
It is an object of the present invention to provide a source follower circuit including a current source improved in the constant current property, laser driving apparatus, semiconductor laser apparatus, current-voltage conversion circuit, and light receiving circuit.
The inventor performed various investigations in order to accomplish the above object and has been accomplished the present invention as follows.
One aspect of the present invention is a source follower circuit. The source follower circuit comprises a first III-V compound semiconductor transistor, a biasing stage, and a current source portion. The first III-V compound semiconductor transistor has its source electrically connected to an output of the source follower circuit, its drain, and its gate for receiving an input signal. The biasing stage has means for generating a first bias voltage at a first node, and means for generating a second bias voltage smaller than the first bias voltage at a second node. The current source portion comprises a second III-V compound semiconductor transistor. The second III-V compound semiconductor transistor has its source, its drain, and its gate for receiving the second bias voltage. A third III-V compound semiconductor transistor has its source, its drain electrically connected to the output, and its gate for receiving the first bias voltage, and is provided between the current source portion and the output.
This source follower circuit may have the following configuration: the third compound semiconductor transistor has a coupling capacitance Cgd3 between the gate and drain thereof and the second compound semiconductor transistor has a coupling capacitance Cgd2 between the gate and drain thereof. The third compound semiconductor transistor is provided such that a value of the coupling capacitance Cgd3 is smaller than that of the coupling capacitance Cgd2.
Another aspect of the present invention is a source follower circuit. The source follower circuit comprises a source follower stage and a biasing stage. The source follower stage comprises first, second, and third compound semiconductor transistors. These transistors are connected in series between a first power line and a second power line. Each of the first, second, and third compound semiconductor transistors has its source, drain, and gate. The biasing stage has first and second nodes and a first circuit portion. A first bias voltage is provided at the first node. A second bias voltage is provided the second node. The first circuit portion is provided to generate the second bias voltage smaller than the first bias voltage.
In the source follower circuit, the first, second and third compound semiconductor transistors are electrically connected as follows: the gate of the first compound semiconductor transistor is coupled to an input of the source follower circuit. The source of the first compound semiconductor transistor is coupled to an output of the source follower circuit. The gate of the second compound semiconductor transistor is electrically connected to the second node. The gate of the third compound semiconductor transistor is electrically connected to the first node. The third compound semiconductor transistor is provided between the source of the first compound semiconductor transistor and the drain of the second compound semiconductor transistor.
Still another aspect of the present invention is a laser driving apparatus. The laser driving apparatus comprises first and second source follower circuits and a differential transistor pair circuit. The differential transistor pair circuit has a pair of compound semiconductor transistors and a current source. Each compound semiconductor transistor has its source, a drain, and a gate, and they are connected with each other so as to constitute a differential pair. The current source is connected to the sources of the pair of compound semiconductor transistors. An output of the first source follower circuit is electrically connected to the gate of one transistor of the pair of compound semiconductor transistors. An output of the second source follower circuit is electrically connected to the gate of the other transistor of the pair of compound semiconductor transistors.
Still another aspect of the present invention is a semiconductor laser apparatus. The semiconductor laser apparatus comprises the laser driving apparatus and a semiconductor laser. The semiconductor laser has an anode and a cathode. The drain of one transistor of the pair of compound semiconductor transistors in the differential pair circuit is electrically connected to one of the anode and cathode of the semiconductor laser. The drain of the other transistor of the pair of compound semiconductor transistors in the differential pair circuit is electrically connected to a reference potential line. The other of the anode and cathode of the semiconductor laser is electrically connected to the reference potential line.
Still another aspect of the present invention is a current-voltage conversion circuit. The current-voltage conversion circuit comprises a preamplifier. The preamplifier has an input, an output, an amplification portion, and a feedback portion. The input is provided to receive a current signal. The amplification portion is provided between the input and output. The amplification portion comprises the source follower circuit. The feedback portion connects the output to the input.
Still another aspect of the present invention is a light receiving circuit. The light receiving circuit comprises a photodiode and the preamplifier. An input of the amplification portion is connected to one of the anode and cathode of the photodiode.
In the source follower circuit as described above, the output voltage of the source follower circuit varies in response to a voltage at the gate of the first transistor. This variation causes the voltage variation at the drain of the third transistor as well. Assuming that the second transistor does not have any influence on the third transistor, the drain current of the third transistor varies according to its drain current characteristics in response to the variation in the drain voltage of the third transistor.
This drain current flows through the second transistor. In order to pass this current through the second transistor, the drain voltage of the second transistor varies according to its drain current characteristics. If the drain current is increased by the output voltage, the increase of the source-drain voltage in the second transistor causes the source-gate voltage of the third transistor to decrease. This decrease reduces the drain current change caused by the output change in the third transistor. Thus, the increase of the drain current caused by the output voltage change becomes smaller than the variation of the drain current in a current source consisting of the second transistor. Therefore, the constant current characteristic of the current source is improved in the source follower circuit. This improvement provides the source follower circuit with the gain increase. On the other hand, when the drain current is decreased by the output voltage, the operation of the source follower circuit can be described in a similar way.
This improved gain increases signal amplitudes at the output of the source follower circuit. This increase in amplitude can reduce the potential difference between the gate and source of the first transistor. However, the first transistor has a coupling capacitance between the gate and source thereof. Current is necessary to charge this coupling capacitance because the coupling capacitance should be charged in response to the output variation. However, the charging current has become smaller because of the reduction of the potential difference between the gate and source.
The third transistor is arranged between the output of the source follower circuit and the second transistor. Because of this arrangement, the coupling capacitance between the output and the drain of the third transistor is smaller than that between the output and the gate of the second transistor. Therefore, the third transistor can decrease a capacitance added to the output of the source follower circuit even when the source follower circuit is provided with a desired current source.